M cells in the epithelium overlying intestinal lymphoid follicles take up and transport soluble antigens, viruses, bacteria, and protozoa from the intestinal lumen into Peyer's patches, where secretory immune responses are generated. The aim of the proposed research is to investigate the role of M cells in the development of mucosal immune responses. A panel of murine monoclonal antibodies with novel specificity for rabbit M cells will be used to conduct systematic studies of M cell function. M cells will be identified by immunoelectron microscopy on the basis of expression of surface molecules. Monoclonal antibody-ricin A chain immunoconjugates will be used to immunotarget M cells in vivo. Two-color fluorescence activated cell sorter analyses will enable the positive identification of isolated M cells, and of M cells after phagocytosis of fluoresceinated latex microspheres. The molecular weights of M cell-surface antigens will be ascertained by immunoprecipitation and Western blotting. The specific recognition by M cells of Peyer's patch lymphocyte subpopulations, and the ability of M cells to convey stimulatory signals to antigen presenting cells and T cells will be investigated in vitro. The acquisition of M cell phenotype by a rabbit intestinal epithelial cell line will be monitored after co-culture with Peyer's patch lymphocyte subpopulations. Establishment of a cell line with M cell properties will be attempted. The function of M cells in the immune response will be established in vitro by examining the ability of M cells to express class II molecules, and to function as antigen presenting cells in the mixed leukocyte culture, mitogen-induced, and antigen-dependent T cell proliferation. The role of M cells in the production of IgA, and in the development of IgA antibody will be determined. These studies will permit design of new mechanisms for mucosal immune defense.